The major histocompatibility complex (MHC) related protein 1, MR1, is highly conserved among mammalian species, particularly the alpha-1 and alpha-2 domains. This feature made MR1 a plausible candidate restriction element for a small population of T cells that express an invariant TCR alpha chain in humans, mice and cattle. Initial studies indicated that these invariant alpha/beta T cells require a beta-2-microglobulin-dependent, TAP-independent MHC class Ib molecule for selection/expansion. We generated mice lacking the alpha-1 and alpha-2 domains of MR1 and demonstrated that they harbor essentially no T cells expressing this invariant TCR alpha chain. Further characterization of these MRl-restricted T cells demonstrated that they are preferentially located in the lamina propria and, therefore, are mucosal associated invariant T (MAIT) cells. Recent biochemical studies suggest that MR1 is only expressed on the cell surface in association with an as yet unidentified ligand. In addition, MAlT cells were found to require B cells and commensal flora for selection and/or expansion in the gut lamina propria. Given this, we think it is likely that a ligand derived from or induced by commensal flora promotes cell surface expression of MR1 on B cells and/or other antigen presenting cells in the gut, which thereby activate MAlT cells. In light of their similarities, MAlT cells may be functionally analogous to NK T cells, which express an invariant TCR alpha chain and rapidly secrete large amounts of cytokines in response to the class Ib molecule CDld in association with glycolipid ligands. This proposal is designed to test the hypothesis that MAlT cells are a distinct population of T cells at the mucosal interface that respond rapidly to MR1 in the presence of commensal flora by secreting cytokines that may promote tolerance. Specifically, we will: 1) evaluate expression of MR1 in vivo using a panel of anti-MR1 monoclonal antibodies that we recently generated; 2) assess mucosal immune function in mice deficient for MR1 and MAlT cells using several models of inflammatory bowel disease and a gastric pathogen; 3) analyze transgenic mice expressing only the MAlT cell invariant TCR alpha chain. These studies will elucidate the role of MR1 and MAlT cells in the immune response.